(1) Field of the Invention
The present invention lies in the field of power plants for rotary wing aircraft, and more particularly rotary wing aircraft having a plurality of engines.
The present invention relates to a power plant for a rotary wing aircraft and to a rotary wing aircraft having said power plant, and it also relates to a method of managing such a power plant.
(2) Description of Related Art
A power plant for a rotary wing aircraft generally comprises one or two engines and a main power transmission gearbox (MGB). Each engine drives the MGB mechanically in order to rotate at least one main outlet shaft of the MGB. The main outlet shaft is constrained to rotate with a main rotor of the rotary wing aircraft in order to provide the aircraft with lift, and possibly also with propulsion.
The MGB generally has secondary outlet shafts, e.g. for driving rotation of a tail rotor or indeed of one or two propulsion propellers via one or more auxiliary gearboxes, and also for driving an electricity generator and/or hydraulic systems. The respective frequencies of rotation of these secondary outlet shafts are generally different from the frequency of rotation of the main outlet shaft.
It should be observed that the term “engine” is used to mean any power unit suitable for driving said MGB mechanically, and consequently suitable for contributing to providing the rotary wing aircraft with lift and/or propulsion via the main rotor. Such engines may for example be turboshaft engines of rotary wing aircraft.
An aircraft may also have an auxiliary power unit (APU). By way of example, the APU may be used for generating electricity or for driving hydraulic systems. However the APU does not drive the MGB, nor does it drive a rotor on a rotary wing aircraft.
Consequently, the APU of an aircraft does not constitute an engine in the meaning of the invention.
It is nowadays common practice to use a power plant on board rotary wing aircraft that comprises a plurality of engines and more particularly two engines, each engine being controlled by a dedicated control unit. The engines are generally identical.
Engines are said to be “identical” when they present characteristics for driving a rotary member, such as frequency of rotation, power, and/or torque that are identical.
Known Documents U.S. Pat. No. 4,177,693 and U.S. Pat. No. 3,963,372 describe respective power plants, each having three engines driving an MGB, all three engines being identical. More particularly, Document U.S. Pat. No. 3,963,372 proposes a solution for managing and controlling the power of its three engines.
Also known is Document WO 2012/059671, which proposes a power plant having two engines with different available maximum powers.
Furthermore, Document US 2009/0186320 describes an aircraft having a power plant with three engines that is used for training pilots and for simulating a total failure of an engine in various flight configurations. That system serves in particular to adapt the power available from a power plant as a function of the total weight of the aircraft.
Also, Document EP 2 724 939 describes a hybrid power plant having at least two fuel-burning engines and one electric motor, the electric motor being capable of compensating for a loss of power from a fuel-burning engine as a result of that engine failing.
Finally, Documents EP 2 623 746, EP 2 623 747, and EP 2 623 748 are known, which describe how to determine a power margin for an engine by performing a health check on that engine.
Within a power plant, regardless of the number of the engines that it includes, each turboshaft engine may operate at an ordinary rating during cruising flight. This ordinary rating is sometimes referred to as its maximum continuous power (MCP) rating, specifying the maximum power the engine can deliver continuously without limit on duration.
Furthermore, each engine may also operate at special ratings that are used during particular stages of operation of the rotary wing aircraft. In particular, during the stage of a rotary wing aircraft taking off, each engine operates at a takeoff rating which associates a maximum takeoff power (TOP) with a duration of use that is restricted. The maximum takeoff power TOP is greater than the maximum continuous power MCP.
In addition, when a power plant has at least two engines, it is overdimensioned so as to enable flight to be performed safely in the event of one engine failing. Specific emergency or “contingency” ratings are used, e.g. on twin-engined aircraft, when one of the engines has failed and thus delivers no power. These emergency ratings are referred to as one engine inoperative (OEI) ratings.
A first emergency rating known as OEI 30 sec thus specifies a supercontingency power that the engine that is still operational can deliver for a duration of about thirty consecutive seconds.
A second specific emergency rating known as OEI 2 min likewise specifies a maximum contingency power that the engine that is still operational can deliver for a duration of the order of two minutes.
A third specific emergency rating known as OEI cont specifies an intermediate contingency power that the engine that is still operational can continue to deliver for a duration lasting to the end of a flight.
A characteristic of an engine may be a guaranteed minimum OEI emergency mechanical power OEIPMIN, i.e. the minimum power that the engine must be capable of delivering in OEI emergency mode. Consequently, the OEI powers associated with each of the emergency ratings OEI 30 sec, OEI 2 min, and OEI cont are all greater than or equal to a guaranteed minimum OEI emergency mechanical power OEIPMIN.
The OEI emergency mechanical powers developed while those specific emergency ratings are in use need to be greater than the powers developed while using the ordinary rating in order to compensate for the power that is lost as a result of one failure of an engine. The guaranteed minimum OEI emergency mechanical power OEIPMIN is thus greater than the maximum continuous power MCP.
Thus, engines need to be overdimensioned in order to comply with safety requirements and to be capable of delivering extra power in the event of an engine failing. The engines then generally co-operate with an engine control unit (ECU).
Furthermore, the use of the OEI emergency ratings for the recommended durations is associated with predefined maintenance operations. Even if making use of these OEI emergency ratings fall within the design specifications of such an engine, the powers that they develop are considerably greater than the maximum continuous power MCP. Consequently, maintenance must be performed after using any of these OEI emergency ratings, in particular in order to check the state of the engine and of its components. In addition, using those OEI emergency ratings for durations longer than the recommended durations can lead to degradations in the engine that then require maintenance operations of greater magnitude.
Consequently, the use of the OEI emergency ratings, which are thus followed by maintenance operations of greater or smaller magnitude, has a direct maintenance cost (DMC) that may be high.
This DMC takes account of all of the costs associated with using and running the engine. This DMC is thus an important element in the cost of operating a rotary wing aircraft.
Furthermore, for an engine, this DMC depends also on the utilization rate of the engine. An engine that is used at 50% of its maximum continuous power MCP presents, for example, a DMC that is less than an engine that is used at 100% of its MCP. This difference in the DMC depends in particular on the stressing of the components of the engine, and more particularly on the rotary components that wear more quickly when the engine is used at high power, thus likewise leading to maintenance operations being performed more frequently.
In addition, an engine may also suffer degradations during its lifetime that have an impact on its characteristics. Such an engine nevertheless continues to deliver power even though that power might be reduced.
By way of example, one kind of degradation that a rotary wing aircraft engine might suffer is the appearance of defects in at least one component of the engine, or else ingestion of a foreign object by the engine. Such ingestion of a foreign object is frequently referred to as foreign object damage (FOD).
Those degradations, although they do not lead to a total loss of the mechanical power delivered by an engine, nevertheless degrade its operation and limit the power it can deliver. For example, the engine may be able to deliver power continuously that is equal to the maximum continuous power MCP, but it may no longer be able to reach, even temporarily, an OEI emergency power that is greater than or equal to the guaranteed minimum emergency mechanical power OEIPMIN.
The engine then requires maintenance in order to repair the degradations it has suffered and in order to enable it once more to deliver OEI emergency powers that are greater than or equal to the guaranteed minimum emergency mechanical power OEIPMIN. Such degradations, and more particularly the maintenance operations used for correcting them, thus increase the DMC of the engine.
Nevertheless, the engine can be still be used in spite of its degradations but with a lower level of power available from the engine. The flight envelope of the rotary wing aircraft may then be reduced in order to take account of this drop in the power available from the engine, while still guaranteeing safe flight. Likewise, the total weight of the rotary wing aircraft is generally reduced in order to take account of this drop in the power available from the engine, with this reduction in the total weight of the aircraft being obtained by reducing the payload it transports.
Finally, an engine also suffers a loss of power as a result of aging. Throughout the lifetime of the engine and its utilization, all of its components suffer wear and they might possibly become deformed as a result of the thermal stresses to which they are subjected. As a result, beyond a certain degree of aging, the engine can no longer deliver, even on a temporary basis, an OEI emergency power that is greater than or equal to the guaranteed minimum emergency mechanical power OEIPMIN for which it is designed.
The maintenance operations performed on the engine throughout its lifetime enable it to be kept in form and enable certain components to be replaced, if necessary, in order to maintain the characteristics of the engine at a satisfactory level. Consequently, the engine operates reliably, but as from a certain degree of aging, it operates with loss in terms of the emergency mechanical power that it is capable of delivering compared with the guaranteed minimum emergency mechanical power OEIPMIN. These maintenance operations also have an impact on the DMC of the engine.
Furthermore, executing these maintenance operations that result from such degradations of the engine takes the rotary wing aircraft out of service, so it is then not available to perform a flight.
In addition, in order to characterize the effects of such degradations and/or of the aging of an engine, each engine is regularly subjected to engine health checks, also referred to as engine power checks (EPCs). These engine health checks serve to monitor the performance of an engine by means of monitoring parameters representative of the degradation of the engine.
An engine health check is thus performed by comparing the performance of the engine with the performance of the same engine as obtained on a test bench and as declared by the manufacturer. An engine health check serves to determine respective margins for one or more monitoring parameters of the engine relative to limit values for each monitoring parameter, and consequently makes it possible to determine the mechanical powers that are available, such as the maximum continuous power MCP and the OEI emergency powers. Furthermore, it is possible to deduce from these monitoring parameters whether the engine has suffered degradations and whether it needs to be subjected to maintenance operations, in particular in order to be capable once more of delivering the mechanical powers for which it is appropriate.
An engine health check thus makes it possible to determine firstly the current characteristics of the engine and secondly whether the engine needs to be subjected to one or more maintenance operations.
For example, if the engine is a turboshaft engine having a high pressure turbine arranged upstream from a free turbine, one monitoring parameter may be the temperature of the gas at the inlet to the free turbine. Another monitoring parameter relating to the power delivered by the turboshaft engine may be the speed of rotation of the gas generator of the engine, which is substantially proportional to the power or to the torque that the engine delivers.
In addition, the monitoring parameters may depend on various criteria such as the speed of rotation of the engine used, engine stabilization conditions, or indeed atmospheric conditions, and each engine health check needs to be performed using a predetermined procedure.
Such an engine health check may be performed while in flight, or between two flights. Furthermore, such an engine health check is performed on a regular basis, e.g. once every twenty hours of engine operation.